Calculate SMT stencil area ratio, aspect ratio, paste volume, maximum foil thickness, or minimum aperture size for rectangular and round openings in mm, mils, µm, or inches.
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Stencil Area Ratio Formula
- AR = area ratio (dimensionless)
- L = aperture length
- W = aperture width
- D = aperture diameter
- T = stencil foil thickness
All inputs must use the same units before the ratio is computed. The calculator converts mm, mil, µm, and inches internally. Aspect ratio (smallest opening divided by thickness) is reported alongside area ratio because both must pass for reliable paste release. IPC-7525B recommends an area ratio of at least 0.66 and an aspect ratio of at least 1.5.
Reference Tables
Use these as starting points. Actual release depends on stencil finish, paste chemistry, squeegee setup, and pad design.
| Area ratio | Expected release | Action |
|---|---|---|
| ≥ 0.75 | Excellent | Standard process |
| 0.66 to 0.74 | Good with laser-cut stainless | Verify with print test |
| 0.55 to 0.65 | Marginal | Use nano-coated or electroformed foil |
| < 0.55 | Unreliable | Reduce thickness or enlarge aperture |
| Paste type | Particle size (µm) | Min opening (5-particle rule) |
|---|---|---|
| Type 3 | 25 to 45 | ~225 µm |
| Type 4 | 20 to 38 | ~190 µm |
| Type 5 | 15 to 25 | ~125 µm |
| Type 6 | 5 to 15 | ~75 µm |
Worked Example and FAQ
Example. A 0.30 mm by 0.25 mm rectangular aperture in a 0.10 mm foil.
- Opening area = 0.30 × 0.25 = 0.075 mm²
- Wall area = 2 × (0.30 + 0.25) × 0.10 = 0.110 mm²
- Area ratio = 0.075 / 0.110 = 0.68 (passes the 0.66 target)
- Aspect ratio = 0.25 / 0.10 = 2.5 (passes the 1.5 target)
What if my area ratio is too low? Reduce stencil thickness, enlarge the aperture (home plate, rounded rectangle), or switch to a nano-coated or electroformed foil that releases at 0.55.
Why does aspect ratio still matter? Aspect ratio governs long, narrow apertures (like fine-pitch QFP leads) where area ratio alone can be misleading. Both checks must pass.
Which target should I pick? Use 0.66 for standard laser-cut stainless steel, 0.55 for nano-coated or electroformed stencils, and 0.75 when you want extra margin on fine-pitch BGA or 01005 work.
